In vehicles today, e.g. urea is used as a reductant in SCR (Selective Catalytic Reduction) systems comprising an SCR catalytic converter, in which catalytic converter the reductant and NOx gas can react and be converted into nitrogen and water. Different types of reductants can be used in SCR systems. For example, AdBlue is one commonly occurring reductant.
One type of SCR system includes a container that contains a reducing agent. The SCR system also has a pump that is arranged to pump the reducing agent from the container via a suction tube and supply it under pressure via a pressurized tube to a dosing unit that is arranged in an exhaust system in the vehicle, such as in an exhaust pipe of the exhaust system that is arranged to conduct exhaust from a vehicle engine to the surroundings thereof. The dosing unit is arranged to inject a required amount of reducing agent into an exhaust system upstream of the SCR catalytic converter according to operating routines stored in a control unit in the vehicle.
The dosing can, for example, occur in a predetermined stroke, such as 1 Hz, in which the dosing unit is open for a given predetermined period of each stroke, whereupon reducing agent can thus be dosed under pressure.
There is a constant need to reduce the amount of emissions from engines in motor vehicles. This applies not least to heavy motor vehicles such as trucks and buses, as legal requirements for increasingly lower emissions are continuously being intensified. It is thus of the utmost importance that SCR systems in vehicles deliver quality function not to degrade performance.
No dosing of reducing agent into the exhaust system occurs in many operating cases for the vehicle, as active regeneration is not necessary all that often. One such operating case can be, for example, when the exhaust temperature of the vehicle is lower than a given predetermined temperature. At such low exhaust temperatures, dosed reducing agent will not react with the NOx gases in the exhaust from the engine. Another such operating case in which no dosing occurs can be when a prevailing temperature of a surface of or component in the exhaust duct from the vehicle engine is lower than a given predetermined temperature. Dosed reducing agent could not be vaporized to the desired extent in this case. Dosed reducing agent can then take the form of undesired deposits or crystals in the exhaust system, which can cause degraded vehicle performance. Measures to remove such deposits are both costly and time-consuming. In the worst case, parts of the exhaust system will need to be replaced, which is associated with very high costs. It should be noted that an aftertreatment system in, for example, a modern heavy vehicle currently accounts for a significant share of the total value of the vehicle.
The dosing unit is thus kept closed in many operating cases because the dosing of reducing agent is quite simply not desirable.
When the reducing agent in the SCR system is pressurized, undesirable leakage of reducing agent from the dosing unit to the exhaust duct can occur for various reasons, including during periods when the dosing unit is closed not to dose reducing agent. Under unfavorable conditions, such leakage can cause crystallization and/or deposits in the exhaust system, which can negatively affect the SCR system.
It is thus desirable to ensure that no or minimal leakage of reducing agent occurs when the dosing unit is closed.
US 20120047880, DE102010030854 and US20100082224 describe various aspects of SCR systems.